AutoFib2+WYFFOS (AF2) is the multi-object, wide-field,
fibre spectrograph working at the Prime focus of the 4.2m William Herschel Telescope (WHT)
. It contains
150 science fibres each of 1.6 arcsec diameter, and 10 fiducial bundles for
acquisition and guiding. At the Prime focus, the fibres are placed onto a
field plate by the robot positioner Autofib2 at user-defined sky coordinates.
Object light is transmitted along fibres 26 metres in length to the Wide
Field Fibre Optical Spectrograph (WYFFOS). The path from Prime focus to the spectrograph
consists of a prism, fibre button, 26 metres of fibre, finger, microlens
and the facet block.
AF2/WYFFOS has received several performance-enhancing
upgrades during 2013A, and is offered to observers for 2014A.
(1) The rms accuracy with which science targets can be
acquired onto fibres has been substantially improved, via:
implementation of a new automatic field-acquisition tool;
better characterisation of geometric distortions in the focal plane;
and an improved algorithm for fibre placement.
(2) A new 4k x 4k red-sensitive CCD with very low-fringing, Red+4, has
been successfully commissioned and is offered as the default
detector for WYFFOS.
(3) New calibration lamps (a QTH continuum lamp and ThAr, Cd
and Zn arc lamps), have been purchased and tested.
In addition, an extensive investigation of the throughput of AF2/WYFFOS
has been carried out, including on-sky throughput measurements with
both the Red+4 and WHTWFC detectors, and microscopic inspection of all
fibre input-end microprisms.
A more detailed description of the recent enhancements and investigations
can be found here.
Teams considering submission of an AF2 proposal are encouraged to
contact the AF2 instrument specialist if they have any queries.
The AF2/WYFFOS multi-fibre spectrograph will be available in 2013B on a shared-risks basis.
Recent work has confirmed that
imperfect acquisition and guiding were contributing significantly to the
low throughput noted by observers. Preliminary tests indicate that new procedures improve the acquisition accuracy to better than 0.4
arcsec rms out to 15 arcmin from the center (the nominal unvignetted maximum field radius is 20 arcmin), and we expect that ongoing planned work will yield further improvements before 2013B.
For reference, an offset of 0.5" leads to ~20% aperture loss from a point source (seeing of 0.5"-1.5" FWHM), when compared to the corresponding flux for a perfectly centered fibre.
We also expect to offer 2013B observers a new low-fringing detector for use with AF2: the 4k*4k REDPLUS CCD.
AF2's availability in 2013B will be on a shared-risks basis because the on-sky tests of the new acquisition software, and the commissioning of the new CCD are not yet complete.
Teams considering submission of an AF2 proposal are encouraged to notify the AF2 instrument specialist.
AF2/WYFFOS is not offered in the semester 2013A, to allow implementation of a number
of upgrades aimed at a
significant improvement in acquisition accuracy and system throughput. The
upgrades are expected to
address the low-throughput issues recently reported by observers.